A computer network is a collection of interconnected computing devices that exchange data and share resources. In a packet-based network the computing devices communicate data by dividing the data into small blocks called packets. Certain devices within the network, such as routers, maintain routing information that describes routes through the network. In this way, the packets may be individually routed across the network from a source device to a destination device. The destination device extracts the data from the packets and assembles the data into its original form. Dividing the data into packets enables the source device to resend only those individual packets that may be lost during transmission.
Virtual private networks (VPNs) are often used to securely share data over a public network, such as the Internet. For example, an enterprise that includes multiple geographically separated sites, each site including one or more computing devices, may establish a VPN to allow the computing devices to securely communicate through the Internet or another public network. In particular, VPNs transport layer three (L3) communications, such as Internet Protocol (IP) packets, between the remote sites via the public network.
In some cases, a VPN may be configured to carry L3 multicast traffic, such as Internet Protocol Television (IPTV), desktop conferences, corporate broadcasts, music and video web casts, and other forms of multimedia content. Multicast VPNs (MVPNs) typically rely on ingress replication to transmit the multicast traffic from a multicast source to subscriber devices within the MVPN sites. Ingress replication causes an ingress router of a MVPN to replicate a multicast data packet of a particular multicast group and send it to each egress router of the MVPN on the path to a subscriber device of that multicast group. However, ingress replication may be a reasonable model only when the bandwidth of the multicast traffic is low and/or the number of replications performed by the ingress router for a particular multicast data packet is small.
In order to handle high bandwidth multicast traffic, a MVPN may utilize protocol independent multicast (PIM) to tunnel multicast packets from a multicast source to subscriber devices within the MVPN sites. However, using PIM for MVPNs introduces fundamental scalability issues when a network includes a large number of MVPNs each with a large number of subscriber sites.
For a particular MVPN, a router maintains PIM neighbor adjacencies with every other router that has a site in that MVPN. Thus for a given router-router pair, multiple PIM adjacencies may be required, one per MVPN that the routers have in common. For each such PIM neighbor adjacency, the router sends and receives PIM “hello” packets transmitted periodically. For example, on a router with 1000 MVPNs and 100 sites per MVPN the router would typically maintain 100,000 PIM neighbors. In this case, a default hello interval of 30 seconds would result in an average of 3,333 hello messages per second.
Furthermore, PIM is a soft state protocol that requires periodic transmission of customer control information, such as PIM join/prune messages. A router propagates the customer join/prune messages received from a subscriber device to other routers in the network. Each router in the network participating in one or more MVPNs periodically refreshes these PIM customer join/prune messages. This can lead to a large overhead of periodic maintenance messages. Lastly, a router may use PIM to setup a multicast tree across the network for each MVPN to which the router belongs. In this way, PIM cause the network to maintain state for each MVPN established across the network.